Process for hydrogen production from water

ABSTRACT

Hydrogen is produced from water by first reacting I 2 , SO 2  and H 2  O to make hydrogen iodide and sulfuric acid. A substantial molar excess of SO 2  and I 2  in the reaction zone creates a lighter sulfuric acid-bearing phase and a heavier polyiodic-acid-bearing phase. The heavier phase is separated, degassed and then contacted with phosphoric acid to permit distillation of HI of low water content and recovery of I 2  as a separate fraction. Hydrogen is recovered from HI vapor, as by thermal decomposition.

This invention relates to the production of elemental hydrogen and moreparticularly to cycles for producing elemental hydrogen from water,especially those which utilize thermochemical reactions.

Certain nuclear reactors built for commercial power generation producelarge quantities of quality (high temperature) heat, which hasheretofore generally been used for the generation of steam to powerturbines that drive electrical generators. Some solar collectors arealso capable of producing sufficient quality heat, and these may alsoserve as sources of energy to thermochemical reactors.

The cost of producing hydrogen in commercial quantities has risensubstantially with recent rises in the price of natural gas andpetroleum feedstocks, from which hydrogen has heretofore generally beenproduced. Thus hydrogen is becoming a more valuable product. Theelectrolytic decomposition of water provides another way of producingelemental hydrogen; however, the high cost of electric power to effectsuch electrolytic decomposition has heretofore made this processeconomically impractical on a very large commercial scale.

In addition to its present chemical uses, hydrogen has often beenmentioned as a possible way of storing energy until needed. By feedingstored hydrogen to fuel cells, or to combustion processes, electricitycan be generated; however, the high cost of hydrogen has heretoforelimited its use in such a fashion.

Various thermochemical cycles have been proposed for the creation ofhydrogen, and from an efficiency standpoint, a number of these appear tobe eminently sensible. However, from a practical standpoint, none ofthese thermochemical cycles has been without its drawbacks. Forinstance, the direct thermal decomposition of water requirestemperatures well above 2000° C., which temperatures appear to rendersuch a process impractical for commercial implementation in the nearfuture.

Several series of chemical reactions have been proposed as cycles forarriving at the end result of creating hydrogen from water; however,these too have not been without their problems. For example, U.S. Pat.No. 3,929,980, issued Dec. 30, 1975 to Abraham et al., utilizes a seriesof steps which involve the reaction between crystalline iodine andmolten potassium nitrate, and after several subsequent steps, itproduces aqueous hydrogen iodide that is ultimately thermally decomposedto produce hydrogen. U.S. Pat. No. 3,839,550, issued Oct. 1, 1974 to R.H. Wentorf, Jr., teaches a closed-cycle thermochemical process forproducing hydrogen by the decomposition of water that is keyed to thereaction of hot liquid iodine with lithium hydroxide to produce lithiumiodide, which is in turn hydrolyzed to create hydrogen iodide, which isultimately thermally decomposed to produce hydrogen. Both of theseprocesses have fairly complex step-wise requirements and have not beenproved to be economically practicable at the present time.

A further process has been proposed for producingthermochemically-decomposable hydrogen iodide as a part of aclosed-cycle which is based upon the Bunsen reaction: 2H₂ O + SO₂ + I₂ →H₂ SO₄ + 2HI. This process is described in detail in copending patentapplication Ser. No. 786,009, filed Apr. 8, 1977. In this cycle, sulfurdioxide, water and iodine are reacted to produce sulfuric acid andhydrogen iodide under conditions which create a two-phase system. Theheavier phase contains the desired hydrogen iodide product, and thelighter phase contains nearly all of the H₂ SO₄. Although this overallprocess appears to have great promise, there is considered to be roomfor improvement in various of the individual steps.

One of the problems in producing H₂ from aqueous HI is that it isdifficult to obtain anhydrous HI because HI forms an azeotrope of about57 wt. percent (W/o) HI so that distillation of aqueous HI ofsubazeotropic composition, i.e., containing less than 57 w/o HI,produces a distillate rich in H₂ O and a residue approaching theazeotropic composition. Thus the production of anhydrous HI is notsimple; however, the thermal decomposition of HI in the presence ofwater vapor requires additional heat in separation and recycling and isthus disadvantageous.

It is an object of the present invention to provide an improved cyclefor the production of hydrogen from water based upon the Bunsenreaction. A further object of the invention is to provide a moreefficient and economically more practicable process for producingelemental hydrogen, using the Bunsen reaction to produce a two-phasemixture, and then carrying out an improved separation of the componentsof the heavier phase. These and other objects of the invention will beapparent from the following detailed description when read inconjunction with the appended drawing of an illustrative flow sheetshowing a presently preferred embodiment of the invention.

By carrying out the Bunsen reaction in the presence of an excess of bothsulfur dioxide and iodine, relative to the amount of water available totake part in the reaction, a two-phase reaction system is created whichis susceptible to liquid-liquid separation. The more dense or heavierphase contains the major fraction of the excess SO₂ and I₂ reactantsplus the major portion of the hydrogen iodide that is produced. Most ofthe hydrogen iodide is present in the form of HI₃ and higherpolyiodides, such as HI₃ and HI₇, which contribute to the distinct phaseseparation. Water serves not only as one of the reactants in the Bunsenreaction but also as a part of the medium wherein the reaction occurs,and both phases contain water.

It has been found that, after separating the two phases and removingunreacted SO₂ from the heavier phase, the addition of phosphoric acid tothis phase will cause precipitation of the excess iodine plus thedissociation of the polyiodide acids. The HI, which is freed from theHI_(x) by the dissociation and precipitation of the iodine, is createdat a high chemical activity because the water of the heavier phase isnow efficiently chemically bound by the H₃ PO₄, and this increase iswitnessed by an increase in the HI vapor pressure. Thus, if distillationand phosphoric acid addition are carried out simultaneously (extractivedistillation), HI may be distilled away at distillate HI--H₂ O ratiosfar exceeding those of the residue, even though the HI--H₂ O ratio ofthe residue is subazeotropic. Precipitation of iodine and distillationof HI thus produces a residue liquid whose predominant components are H₃PO₄ and H₂ O. This can be further heated, distilling out the H₂ O andthus reconcentrating the H₃ PO₄. Thus the H₃ PO₄ treatment facilitatesthe separation of HI_(x) solutions into the components I₂, HI and H₂ O.

The abbreviated chamical equations describing the overall thermochemicalcycle, which is based upon the Bunsen reaction, are as follows:

    2H.sub.2 O + SO.sub.2 + I.sub.2 → H.sub.2 SO.sub.4 + 2HI (A)

    h.sub.2 so.sub.4 → h.sub.2 o + so.sub.2 + 1/2o.sub.2 (b)

    2hi → i.sub.2 + h.sub.2                             (c)

the reaction products, sulfuric acid and hydrogen iodide, which resultfrom the Bunsen reaction (A), are not separable by simple distillationtechniques. However, if this reaction is carried out under appropriateconditions, wherein both an excess of sulfur dioxide and an excess ofiodine are present, a two-phase system is created consisting of twodistinct water-containing phases that can be efficiently separated. Ofcourse, the Bunsen reaction is a reversible reaction which does not goto completion, and the equilibrium constants and kinetic considerationsdetermine the extent to which the reaction proceeds. This fact, coupledwith the fact that water is both a reactant and a solvent for bothphases, renders precise definition of the relative amounts somewhatdifficult. In general, although dependent somewhat upon the temperature,up to about 10 percent of the water which is present can take part inthe reaction regardless of the excess amounts of SO₂ and I₂, with theremainder of the water fulfilling its function as a solvent component.Thus, it is appropriate to base any stoichiometric calculations uponsuch a 10 percent figure.

Sulfur dioxide must be present in excess to aid in driving the reactionto the right, and in order to facilitate the presence of the desiredamount of sulfur dioxide in the reaction mixture, the reaction may becarried out under greater than atmospheric pressure. However, reducedtemperatures, e.g., down as low as about -60° C., might be used shouldit be desired to provide increased concentrations of SO₂ while remainingat atmospheric pressure. In general, operation at room temperature(i.e., about 21° C.) and atmospheric pressure maintains the sulfurdioxide at a sufficient chemical activity in the liquids as toeffectively drive the reaction to the right. However, operation somewhatabove the melting point of iodine (114° C.) may also be advantageous.Thus overall, the use of temperatures between about -30° C. and about150° C. is generally contemplated.

Gaseous SO₂ may be supplied by bubbling it through the liquid systemwhere the reaction occurs, and it is convenient to simply carry out thereaction under saturation conditions with SO₂ at about atmosphericpressure. Under these conditions and room temperature, sulfur dioxidedissolves in the aqueous solution in a molar amount more than thestoichiometric amount of water (based upon 10 percent of the totalamount of water present). Moreover, under these conditions additionalSO₂ is continuously available to dissolve in the solution to replacethat which reacts to form H₂ SO₄.

As earlier indicated, iodine is also supplied in an excess amount (alsobased upon 10 percent of the water), and preferably iodine is suppliedin an amount approaching the saturation concentration of iodine in thesolutions. In addition to driving the Bunsen reaction to the right bythe law of mass action, iodine complexes with the hydrogen iodidereaction product and creates hydrogen polyiodides (HI₃, HI₅, HI₇ etc.)and ultimately the distinct two-phase liquid system, thus furthercontributing to the efficiency of the system. If the reaction is carriedout at room temperature and at a ratio of 0.5 gram of I₂ for each gramof H₂ O, a single phase reaction system results that has thecharacteristic yellow color of the HI.SO₂ complex. However, as theamount of I₂ is increased, the liquid system takes on a darker colorwhich is indicative of the presence of the polyiodides. The polyiodideacids, which are sometimes herein referred to as HI₃, are considered tobe complexes wherein the HI and the I₂ have a stronger attraction foreach other than exists between the HI and the SO₂.

When the I₂ level reaches about 1.8 grams per gram of H₂ O, phaseseparation begins to occur; and above about 2 grams, a substantialseparation of the two liquid phases (and thus a separation of thesulfuric acid and hydrogen iodide reaction products) is accomplished.The affinity of these polyiodide acids for water and the fact that thecomplexes thus formed with water reject the sulfuric acid solution arebelieved to account for the formation of the lighter phase whichseparates, with the chemical reaction continuing mainly in the heavierphase as all of the reactants are there at high concentrations. Thereaction may be carried out as a batch reaction, but it is preferablyrendered continuous as described hereinafter.

After separation of the heavier phase from the lighter phase, an initialfiltering of the heavier phase may be performed to remove any solids,e.g., any undissolved iodine, that might possibly be present. Next, theheavier phase is degassed to remove the sulfur dioxide that did notreact; thereafter, it is treated with phosphoric acid. Although the term"phosphoric acid" is generally used throughout this application, theterm should be understood to include H₃ PO₄ alongside H₄ P₂ O₇, P₂ O₅and other such dehydrated species, as well as aqueous solutions of H₃PO₄.

The dehydrating properties of phosphoric acid depend upon the H₃ PO₄concentration, which may vary from about 50 weight percent to about 110weight percent or more, and this has an important effect upon itsinteraction with the heavier phase. It has been found that, as a resultof phosphoric acid addition and its interacting to bind the water, asubstantial amount of iodine precipitation occurs. Depending upon thetemperature, iodine separates as either a liquid or a solid. The endresult is a breakdown of the complexing between the iodine and the HI(which originally created the HI₃ and other higher polyiodides), and theprecipitation of iodine occurs along with the formation of HI as theprevalent hydrogen iodide species. This breakdown is favored by theremoval of the HI according to the laws of chemical equilibrium.

The phosphoric acid treatment may, if desired, be effected in two steps.In the first step the heavier phase is mixed with phosphoric acid so asto form two phases, one rich in iodine the other rich in HI, and thesetwo phases are separated. This operation may be conducted either belowthe melting point of iodine or above it, e.g., between about roomtemperature and about 150° C. However, higher temperatures requiresuperatmospheric pressure to prevent undue volatilization of somecomponents. In the second step, the HI-rich phase is distilled toremove, as an overhead stream, HI which contains little, if any, water.The phosphoric acid solution remaining from the second step isconcentrated by distillation for reuse in the first step.

A more complete and more convenient separation of the components isobtained in a continuous treatment method wherein distillation of the HIproceeds along with the precipitation of the iodine as explainedhereinafter. The temperature is preferably maintained above the meltingpoint of I₂, viz. about 115° C., and it must be high enough to causeboiling and distillation of the aqueous phase containing phosphoric acidand HI. Temperatures as high as 150° C. or even up to about 250° C. maytherefore be employed depending on the H₃ PO₄ concentration and thepressure.

The amount and concentration of phosphoric acid used depends on a numberof factors, such as the completeness of separation desired, the amountof recycle allowed, the desired degree of dehydration of the HI, and theflow scheme employed. In general, in order to obtain substantiallyanhydrous HI, the concentration of H₃ PO₄ in the stream being suppliedshould exceed 80 w/o and preferably exceed 90 w/o. It has been foundthat addition of a sufficient amount of phosphoric acid will so reducethe activity of the water that the vapor pressure of the HI will exceedthe vapor pressure of the water at all HI/H₂ O ratios. In order to expelby distillation the major part of HI, this concentration of H₃ PO₄ atthe point where it is most dilute, i.e., in the bottoms, should not fallbelow 60 w/o and preferably not below about 80 w/o. Because, per pass,the phosphoric acid can only remove from the heavier phase the amount ofwater corresponding to the difference between its most concentrated andmost dilute state in the cycle, in order the reduce the rate of cyclingof the H₃ PO₄, it is clear that there are advantages to using acid of100 w/o or more concentration. This is balanced however by theincreasingly high temperatures required to subsequently dehydrate H₃ PO₄as its concentration increases.

A continuous method of carrying out the invention is depicted in theaccompanying FIGURE wherein the main reaction, in accordance with theBunsen equation, is carried out in a first reactor 9. Reactionconditions are maintained by metering in the water and iodine at thedesired rates and by continuously supplying SO₂ so as to maintain thereaction mixture saturated in SO₂ at one atmosphere pressure and 90° C.As previously indicated, a two-phase liquid system is created. Thelighter and heavier phases are continuously separately drawn off, anddifferent treatments of each are effected.

The lighter phase is preferably heated in a stripper 11 to first driveoff most of the sulfur dioxide, water and iodine, for example byemploying a temperature of about 200° C. at atmospheric pressure. Thesevapors are condensed and recovered for reuse. Non-atmospheric pressuresmay be used, if desired, to substantially change the temperature. Thedegassed aqueous sulphuric acid stream is then supplied to an evaporator13 to separate the remaining water (along with any residual iodine).Thereafter, the sulfuric acid is then heated in a vaporizer 15 at atemperature of at least about 335° C. (which is its boiling point at 1atm.).

The sulfuric acid vapors are then treated, in a known manner, so as totransform the acid into water and sulfur trioxide, which in turn breaksdown, generally in the presence of a catalyst, to sulfur dioxide andoxygen at high temperatures. See for example, U.S. Pat. No. 3,888,750,issued June 10, 1975 to Brecher et al., wherein the thermochemicaldecomposition of sulfuric acid with the resultant production of oxygen,sulfur dioxide and water is illustrated and described in detail.Preferably, the sulfuric acid vapors are transferred to a decompositionreactor 17 wherein a temperature between about 400° C. and about 950° C.is maintained and wherein the sulfuric acid is catalytically decomposedto produce water, sulfur dioxide and oxygen. The water and sulfurdioxide are recycled to the main reactor 9, and oxygen may be recoveredas a by-product.

The heavier phase is preferably passed first to a stripper 19 whereexcess sulfur dioxide is removed by subjection to a vacuum and/or mildheating to increase its vapor pressure. The resultant degassed stream isthen conducted to an extractive distillation column 25 where it isintroduced on a plate generally near the middle of the column. Thebottom of the column is maintained at about 130° C. or above.Concentrated phosphoric acid is supplied to the top of the column 25 andbecomes enriched in water as it flows down the column 25, whereas thevapor, which is flowing countercurrently up the column, becomes more andmore enriched in hydrogen iodide. In addition, the action of thephosphoric acid in reducing the chemical activity of the water has theeffect of causing iodine to precipitate from the aqueous solution atlocations below the plate at which the stream is introduced. Thus, thebottoms from the column 25 include dilute phosphoric acid and iodine astwo nearly immiscible fractions or layers. Depending upon the way thecolumn is operated, and in particular upon the concentration ofphosphoric acid at this point, the bottoms may be substantially free ofHI or may contain a significant amount dissolved in the aqueous layer.Preferably, the column is operated so that the one bottom fractioncontains less than about 0.1 w/o of HI, based upon H₂ O, H₃ PO₄ and HI.The iodine layer is separated from the remainder of the bottoms, and itmay then be cleaned up in a separator 23, where minor amounts ofcontaminants are removed and returned to the column 25 with the streamfrom the stripper 19, before being recycled to the main reactor 9.

The operation of the column 25 is such that the ratio of water to HI inthe dilute phosphoric acid reaching the bottom of the column is higherthan the azeotropic H₂ O/HI weight ratio (about 0.75). The bottoms fromthe column 25 are then circulated to a second column 27 wherein thephosphoric acid is reconcentrated for return to column 25. The overheadstream from column 27 may or may not contain a significant amount of HIdepending on the operation of column 25. If the amount of HI in theoverhead stream is negligible, this overhead stream is basically waterwhich may be returned to reactor 9. If the amount HI is significant butof a definite subazeotropic proportion, the overhead stream is directedto a still 28 which generates an overhead stream of essentially purewater for return to reactor 9 plus bottoms in which the HI isconcentrated to about its azeotropic concentration. The bottoms arereturned to the extractive distillation column 25 at the appropriatelevel.

If it is desired to precipitate some of the iodine prior to extractivedistillation, a precipitator (shown in dotted outline) may be providedin which the heavier phase is treated with either some freshconcentrated phosphoric acid or with a sidestream from column 25 whichis rich in phosphoric acid. The precipitated iodine is sent to separator23, while the remainder becomes the stream which is introduced into theextractive distillation column 25.

The distillate from column 25 has a very high percentage of hydrogeniodide and may be further dried, if desired, to even further reduce theminor percentage of water. The HI is then transferred to a hydrogenrecovery installation 31 where it is treated to recover hydrogen (eitheras molecular hydrogen or as a hydrogen-containing compound) and iodine,for example, by heating to the appropriate temperature for thermaldecomposition, which may be from 114° C. to 500° C. or more depending onthe system and whether a catalyst is used, or by decomposing in someother manner such as photolytically as described in U.S. Pat. No.3,995,016, issued Nov. 30, 1976 to P. A. Kittle. The very top of column25, if desired, need not contain any phosphoric acid but could act as asimple rectifying section wherein water is stripped from thesuper-azeotropic hydrogen iodide. The reflux stream must be cooled tolow temperatures in this case. However, preferably, the concentratedphosphoric acid (at least about 90 w/o H₃ PO₄) which is obtained fromthe bottoms of the column 27 is introduced at the very top of the firstdistillation column, as previously indicated and as illustrated. Thisconcentrated phosphoric acid stream can also be used to supply all orpart of the phosphoric acid-rich liquid which may be supplied to aprecipitator.

As an example of the practicality of the chemistry involved, a batchreaction is carried out using about 300 grams of water, about 200 gramsof sulfur dioxide and about 880 grams of iodine, at atmospheric pressureand at a temperature of about -10° C. The reaction proceeds readilyunder these conditions, and the formation of two distinct phases isobserved. The lower, heavier phase is generally very dark in color, anda sample of it is drawn off, filtered and degassed. Analysis of thedegassed heavier phase shows that it contains about 20 w/o HI, about 20w/o water and about 60 w/o iodine.

A sample of about 14 grams of the degassed heavier phase is combinedwith about 33 grams of 100 percent H₃ PO₄, and a wet precipitate isseparated which weighs about 13.9 grams. Analysis shows the wetprecipitate to contain about 51.2 percent iodine, about 2.9 percent HI,about 41 percent H₃ PO₄, and the remainder water. This constitutesnearly 85 percent of the recoverable I₂.

A slightly less concentrated HI sample than that from the aboveseparation procedure, e.g., one having an H₂ O to HI weight ratio ofabout 2.48:1, is subjected to distillation in an Othmer still having asingle plate which condenses and refluxes all of the distillate. Thestill is operated for a sufficient time to approach steady-statecomposition at a temperature of about 185° C. Sampling of the materialfrom the upper condenser shows that the distillate contains about 54.7percent HI, about 0.7 percent I₂ and the rest water. A sample of thebottoms from the pot shows that the sample analyzes about 91.3 percentH₃ PO₄, about 2.5 percent HI, and the remainder water. Thus it is shownthat a liquid having a water to HI weight ratio of about 2.48, afterdistillation in a one-plate still, produces a distillate whose water toHI ratio is about 0.81 -- equal to an improvement by about a factor of3. Further studies show that increasing the amount of phosphoric acidused results in a reduction to a ratio below the azeotropic ratio of0.75. Studies also show that similar ratio reductions, upondistillation, can be accomplished if proportionately greater amounts ofless concentrated H₃ PO₄, e.g., 85 w/o H₃ PO₄, are substituted for the100 percent H₃ PO₄.

The above demonstrates the effectiveness of H₃ PO₄ in not onlyprecipitating excess iodine from the degassed heavier phase, but in alsocreating a resultant liquid which can be distilled to recover HI.Providing concentrated H₃ PO₄ at the top of the still into which thesupernatent from a precipitator is fed achieves an extremely effectiveseparation of HI via extractive distillation. Thus, the invention isconsidered valuable in recovering HI from a solution consistingessentially of polyiodic acids and water of subazeotropic proportions,regardless of its genesis. "Consisting essentially of" is meant toindicate that no other components are included which would form anazeotrope with either water or HI, and "subazeotropic" means a w/o of HIless than 57 percent. Substantially anhydrous HI, for purposes of thisapplication, is considered to mean HI containing no more than about 5w/o H₂ O.

Although the invention has been described with respect to certainpreferred embodiments, it should be understood that modifications aswould be obvious to one having the ordinary skill of the art may be madewithout deviating from the scope of the invention which is defined bythe appended claims. Various of the features of the invention are setforth in the claims which follow.

What is claimed is:
 1. In a process for the production of hydrogen fromwater, wherein H₂ O, SO₂ and I₂ are reacted in the presence of an excessamount of I₂ and SO₂ to produce sulfuric acid and hydrogen polyiodidesin the form of a two-phase liquid system and wherein the heavier phasecontaining the major portion of the hydrogen polyiodides, which arecomplexes of HI and I₂, is separated from the two-phase liquid system,degassed to remove unreacted SO₂ and treated for hydrogen recovery, theimprovement comprisingtreating said separated heavier phase withphosphoric acid that contains at least about 50 weight percent H₃ PO₄ tobind H₂ O in said separated phase and precipitate I₂ therefrom anddistilling said phosphoric acid-treated liquid phase to separatehydrogen iodide therefrom and passing said separated hydrogen iodide toa hydrogen recovery zone.
 2. The invention in accordance with claim 1wherein said separated hydrogen iodide comprises substantially all thehydrogen iodide initially present in said heavier phase.
 3. Theinvention in accordance with claim 1 wherein said distillation iscarried out at a temperature between about 115° C. and about 250°C. 4.The invention in accordance with claim 1 wherein an initialprecipitation and separation of I₂ is carried out prior to said hydrogeniodide distillation.
 5. The invention in accordance with claim 4 whereinsaid precipitation is carried out at a temperature between about ambientand about 150° C.
 6. The invention in accordance with claim 1 whereinphosphoric acid is added during said distilling step in an amount and ata concentration sufficient to create an aqueous fraction containing atleast 50 w/o H₃ PO₄ following said separation of hydrogen iodide.
 7. Theinvention in accordance with claim 3 wherein phosphoric acid is addedduring said distilling step in an amount and at a concentrationsufficient to create an aqueous fraction containing at least 80 percentH₃ PO₄ following said separation of hydrogen iodide.
 8. The invention inaccordance with claim 7 wherein said aqueous fraction is substantiallyfree of hydrogen iodide.
 9. The invention in accordance with claim 6wherein said process is continuous and wherein the H₃ PO₄ bottoms fromsaid distillation step are separated from precipitated iodine and aresubsequently distilled to remove water and recover concentratedphosphoric acid, which is returned to said hydrogen iodide distillationstep.
 10. The invention in accordance with claim 1 wherein said heavierphase is subjected to extractive distillation in the presence ofsufficient phosphoric acid to produce HI containing less than 5 w/owater plus two bottom fractions, one fraction containing H₃ PO₄ pluswater and HI in subazeotropic proportions and the other fractioncontaining iodine, subjecting said one bottom fraction to furtherdistillation to produce substantially pure water, concentrated H₃ PO₄and a sidestream containing an azeotropic concentration of hydriodicacid, and returning said sidestream to said extractive distillationstep.
 11. A process for the thermochemical production of hydrogen fromwater comprising the steps ofreacting H₂ O, SO₂ and I₂ in the presenceof an excess amount of I₂ and SO₂ to produce sulfuric acid and hydrogenpolyiodides and create a two-phase liquid system wherein the heavierphase contains the major portion of said hydrogen polyiodides, which arecomplexes of HI and I₂, separating the heavier phase from the two-phaseliquid system, degassing said heavier phase to remove unreacted SO₂,subjecting said degassed liquid to extractive distillation in thepresence of phosphoric acid that contains at least about 50 w/o H₃ PO₄so as to remove a substantial fraction of the HI present as gaseous HIin an overhead stream, to precipitate I₂ as one fraction of the bottomsand to create another bottom fraction containing aqueous phosphoricacid, decomposing said gaseous hydrogen iodide to recover hydrogen andiodine, and processing said lighter phase to produce SO₂, H₂ O and O₂.12. The process of claim 11 wherein said extractive distillation iscarried out so that said other bottom fraction contains not more than0.1 w/o HI based upon H₂ O, H₃ PO₄ and HI.
 13. The process of claim 11wherein said other fraction is further separately distilled and the HIcontained in said other fraction is recovered as an aqueous solution ofsubstantially azeotropic composition and returned to said extractivedistillation step.
 14. The process of claim 13 wherein said HI recoveryis effected by a two-stage distillation wherein the first stageseparates a concentrated phosphoric acid bottom fraction for return tosaid extractive distillation and an aqueous HI overhead and wherein thesecond stage separates water as overhead and said aqueous solution ofsubstantially azeotropic composition as the bottoms.
 15. A process forseparating the components of a liquid consisting essentially of iodineand HI in the form of polyiodic acids and water in subazeotropicproportions, which process comprisestreating said liquid with phosphoricacid that contains at least about 50 weight percent H₃ PO₄ so as toprecipitate and separate iodine therefrom and reduce the activity of thewater therein to the point that the vapor pressure of the HI exceeds thevapor pressure of the water and subsequently distilling said treatedliquid to produce gaseous HI.
 16. The process of claim 15 wherein saiddistilling includes extractive distillation in which gaseous HI isproduced as overhead and an I₂ -rich phase is produced as one fractionof the bottoms.
 17. The process of claim 15 wherein said phosphoric acidhas a concentration equal to at least 85 w/o H₃ PO₄.
 18. The process inaccordance with claim 16 wherein said extractive distillation stepemploys concentrated phosphoric acid and wherein the H₃ PO₄ bottomfraction from said extractive distillation step is further separatelydistilled to provide concentrated phosphoric acid for use in an earlierstep.
 19. The process of claim 16 wherein said extractive distillationis carried out at a temperature of at least about 115° C. and in amanner so that the other bottom fraction contains not more than 0.1 w/oHI based upon H₂ O, H₃ PO₄ and HI.
 20. The process of claim 16 whereinthe other bottom fraction is further separately distilled and the HIcontained in said other fraction is recovered as an aqueous solution ofsubstantially azeotropic composition and returned to said extractivedistillation step.